1. Field of the Invention
The invention relates to the process of fabricating semiconductor chips. More specifically, the invention relates to a method and an apparatus for using a second exposure to assist a PSM (phase shifting mask) exposure in printing a tight space adjacent to a large feature during an optical lithography process for manufacturing a semiconductor chip.
2. Related Art
Recent advances in integrated circuit technology have largely been accomplished by decreasing the feature size of circuit elements on a semiconductor chip. As the feature size of these circuit elements continues to decrease, circuit designers are forced to deal with problems that arise as a consequence of the optical lithography process that is typically used to manufacture integrated circuits. This optical lithography process begins with the formation of a photoresist layer on the surface of a semiconductor wafer. A mask composed of opaque regions, which are generally formed of chrome, and light-transmissive clear regions, which are generally formed of quartz, is then positioned over this photoresist layer. (Note that the term xe2x80x9cmaskxe2x80x9d as used in this specification is meant to include the term xe2x80x9creticle.xe2x80x9d) Light is then shone on the mask from a visible light source, an ultraviolet light source, or more generally some other type of electromagnetic radiation together with suitably adapted masks and lithography equipment.
This image is reduced and focused through an optical system containing a number of lenses, filters and mirrors. The light passes through the clear regions of the mask and exposes the underlying photoresist layer. At the same time, the light is blocked by opaque regions of the mask, leaving underlying portions of the photoresist layer unexposed.
The exposed photoresist layer is then developed, through chemical removal of either the exposed or non-exposed regions of the photoresist layer. The end result is a semiconductor wafer with a photoresist layer having a desired pattern. This pattern can then be used for etching underlying regions of the wafer.
Printing Problems with Large Features
As integration densities continue to increase, it is becoming desirable to use phase shifters to define more and more features within a layout. This can lead to problems in some situations. For example, the upper portion of FIG. 1 illustrates the phase shifters for a dark field alternating aperture phase shifting mask 102. The phase shifters (shown with slanted lines to indicate phase) would be set on a dark field, e.g. chromium. The white spaces between the phase shifters correspond to the intended, or original, layout. An aerial image 108 of the use of the PSM mask 102 along with a complementary trim mask (not shown) is in the bottom half of FIG. 1. The dark black regions of the aerial image are areas that received minimal exposure to light and correspond to what the printed wafer will look like. A comparison of the aerial image 108 and the PSM mask 102 can be instructive as to the intended layout.
The PSM mask 102 is being used to define a large feature 104, e.g. a contact landing pad. In this example, the distance between neighboring phase shifters 106 and 107 across feature 104 is large enough to degrade the effectiveness of phase shifting in defining the boundaries of feature 104. This causes a problem in reliably printing the tight space 105 adjacent to large feature 104, as is indicated in by the bridging that is visible in the aerial image 108.
What is needed is a method and an apparatus that facilitates reliably printing a tight space adjacent to a large feature that is defined by phase shifters.
One embodiment of the present invention provides a system that uses an exposure through a second mask to assist an exposure through a phase shifting mask in printing a tight space adjacent to a large feature. During operation, the system exposes a photoresist layer on the surface of a semiconductor wafer through the phase-shifting mask. This phase-shifting mask includes phase shifters that define a space between a first feature and a second feature, wherein the first feature is so large that the effectiveness of phase shifting is degraded in defining the space. Moreover, the degradation in phase shifting and the tightness of the space cause the space not to print reliably when exposed through the phase shifting mask alone. To alleviate this problem the system exposes the photoresist layer through the second mask, wherein the exposure through the second mask assists in exposing the space between the first feature and the second feature so that the space prints reliably.
In a variation on this embodiment, the second mask includes an opening located over the space to assist in exposing the space on the photoresist layer. Note that this second exposure can be sub-resolution.
In a variation on this embodiment, the second mask additionally includes one or more additional assist features, which are not located over the space, yet assist in printing the space. Note that these assist features can be either in-phase or out-of-phase with the light passing through the opening in the second mask.
In a variation on this embodiment, if the second feature is a critical dimension feature, the opening in the second mask is offset from the second feature, so that alignment problems between the phase shifting mask and the second mask do not affect printing of the critical dimension feature.
In a variation on this embodiment, the system additionally performs an optical proximity correction (OPC) operation for segments on the phase shifting mask only.
In a variation on this embodiment, the system additionally performs an OPC operation for segments on the second mask only.
In a variation on this embodiment, the system additionally performs an OPC operation for segments on the phase shifting mask as well as segments on the second mask.
Note that the system may apply OPC to the second mask in a number of ways. (1) The system may perform an OPC operation only for segments on the second mask that abut an original layout. (2) The system may perform an OPC operation for any or all segments on the second mask.